Research On Rotordynamic Instabilities Induced By The Crown Clearance Flow Of A Francis Turbine Runner

With the developments of economy and society, the hydropower has developed rapidly in recent years. Hence with the increase in the capacity and the dimension of the hydroelectric generating set, the structure becomes more and more complicated. As a result, the problem of stability of a generating set is more and more prominent.Much attention has been paid on the lateral vibration of the hydroelectric generating set, especially the Francis turbine, for a long time. When the load exceeds a certain value, a severe flexural vibration will occur, and the amplitude will increase constantly. Furthermore, it can result in the vibration even destroy of the power house. After detailed checking, it was shown that this vibration is a kind of self-excited vibration which caused by the fluid force and moment acting on the backshroud of the runner associated with the leakage flow through the back chamber. As a result, based on the actual vibration problem, model tests were designed on the fundamental characteristics of the flow field in the crown clearance of a Francis turbine runner in whirling and precession motions, respectively. And so the fluid forces and moments under various conditions were measured. The numerical simulation model was established to calculate the characteristics of the flow field in the clearance and the fluid forces and moments. The self-excited effect of the whirling moment and precession moment was analyzed. The stability of a cantilevered shaft-disk system subjected to each fluid moment was discussed. These conclusions can provide the material and the reference of this kind of self-excited vibration of the Francis turbine.Chapter one is the introduction section. According to the research status of the topic on the vibration induced by the fluid of the Francis turbine at home and abroad, the background and the significance of this research topic are put forward. The home and abroad research status and progress of vibration excited by the seal or clearance flow are introduced. The advance of the vibration induced by the clearance flow in the aspects of whirling and precession motions are concluded and summarized, respectively.The second chapter is about the model test. The experimental apparatus, the selection of the parameters and the methodology of the measurement of the experiment were introduced. The calibration test of the four-axis force sensor dynamometer was conducted to get the transfer matrix between the force acting on the end of the cantilevered shaft and the output of the digital scope. The inertia test was also conducted theoretically and experimentally, and the results of the calculation and experiment were compared and validated. The method and the content of the fluid force measurement were generally introduced.The third chapter is about the numerical simulation. Based on the simplified Navier-Stokes equations and the geometry of the crown clearance of the Francis turbine runner, the numerical bulk flow model was established to calculate the steady and unsteady flow field characteristics in the crown clearance and the fluid forces and moments on the crown of the runner in whirling and precession motions. The solutions of the steady and unsteady components were discussed. The computational program of the bulk flow model based on the computer language Fortran was made for calculation.The fourth chapter is about the self-excited effect of the whirling moment acting on the crown of the Francis turbine runner. The steady pressure experiments and calculations under various conditions in the crown clearance of the Francis turbine runner were carried out in order to study about the fundamental characteristics of the steady flow field distribution. The stability analysis model of a cantilevered shaft disk system subjected to the whirl moment was established through the structural coupling between the linear and angular displacement. The criterion for determining the self-excited effect of the whirl moment was obtained. Through the comparison of the experimental result and the computational result of the whirl moment under various leakage flow rates, various preswirl velocities at the inlet of the radial clearance and various widths of the axial clearance, the self-excited effect of the whirl moment and the unsteady flow filed distribution were discussed.The fifth chapter is about the self-excited effect of the precession moment acting on the crown of the Francis turbine runner. The characteristics of the steady flow field in the clearance in precession motion were studied by model test and numerical simulation. Through the comparison of the computational and the experimental results, the self-excited effects of the precession moments were analyzed. The effect of the leakage flow rate in the clearance, the preswirl velocity, the axial clearance and the rotation of the disk on the precession moment were examined, respectively. The unsteady flow field distributions in the clearance in precession motion were discussed under various conditions.The sixth chapter is about the instability analysis of the cantilevered shaft disk system under the fluid force moments. The rotordynamic coefficients used in the rotordynamic instability analysis were obtained by curve fitting the experimental results of the whirling and precession moments, respectively. The effect of the coefficients on the instability was discussed. Through establishing a lumped parameter model subjected to the whirl and/or the precession moment(s) by using Matlab software, the instabilities of the system were discussed and analyzed.